1. Field of the Invention
This invention relates to ultrasound measurement technology, and more particularly, to an ultrasound method and system which is capable of measuring the velocity of a flow, such as the blood flow in a human body, in a non-contact manner through ultrasound means. In practical applications, the invention can be utilized, for example, in the field of physiological diagnosis for blood-flow velocity measurement to determine whether the patient suffers from blood vessel disorders, such as embolism and aneurysm.
2. Description of Related Art
The Doppler effect is a widely utilized natural law in many various scientific and technical applications. The Doppler principle states that when a wave, either sound or radio, is scattered back from a target with relative motion to the source, the observed frequency of the scattering wave is higher than the source frequency if the target is moving toward the source, and lower than the source frequency if the target is moving away from the source. Accordingly, the velocity of a moving target can be detected by directing a wave at the target and then measuring the observed frequency of the scattering wave from the target.
The Doppler effect is a well known principle to all learned people in the field of science and technology, so detailed description thereof will not be further given. In the filed of physiological diagnosis, for instance, the Doppler effect can be used for the measurement of blood-flow velocity in human blood vessels. If the blood, low velocity at a particular point in a blood vessel is abnormal, it can be concluded that patient may be suffering from blood vessel disorders, such as embolism or aneurysm.
FIG. 1A is a schematic diagram used to depict the application of the ultrasound Doppler effect for blood-flow velocity measurement. As shown, an ultrasound beam 10 is controllably directed at a selected measurement point P in a human blood vessel 20. In accordance with the Doppler equation: ##EQU1## where .function..sub.d is the mean value of all the frequency components in the Doppler spectrum (for reasons that will be explained later, the peak-intensity frequency rather than the mean frequency will be used by the invention);
.nu. is the blood-flow velocity at the measurement point P; PA1 .lambda. is the wavelength of the ultrasound beam 10; and PA1 .theta. is the Doppler angle between the emitting direction of the ultrasound beam 10 and the blood-flow velocity vector .nu. at the measurement point P. PA1 .lambda. is the wavelength of the ultrasound beam; PA1 W is the diameter of the circular emitting plane of the ultrasound beam; PA1 F is the focusing length of the ultrasound beam (i.e., the distance between the focal point and the emitting plane of the ultrasound beam); and PA1 .theta. is the Doppler angle between the ultrasound beam and the blood-flow velocity vector.
If the resulted Doppler spectrum is symmetrical in form; the mean frequency .function..sub.d of the Doppler spectrum would be equal to the peak-intensity frequency .function..sub.peak (also referred to as energy peak frequency); whereas, if asymmetrical, the peak-intensity frequency .function..sub.peak would represent the velocity of the majority of the particles in the flow. Since the Doppler spectrum would be asymmetrical in most cases, the peak-intensity frequency .function..sub.peak rather than the mean frequency .function..sub.d would be the predominant frequency in the Doppler equation. Therefore, Eq. (A1) can be rewritten as: ##EQU2##
The relationship of Eq. (A2) is herein and hereinafter referred to as "Doppler-Spectrum Peak-Intensity Frequency Equation" throughout this specification.
From Eq. (A2), it can be deduced that ##EQU3## Accordingly, the magnitude of the blood-flow velocity .nu. can be determined if the values of .function..sub.peak, .lambda., and .theta. are all known. The value of .function..sub.peak can be acquired from the Doppler spectrum, and the value of .lambda. is inherent to the ultrasound beam 10 being used and thus can be known in advance.
A conventional method for determining the Doppler angle .theta. involves the use of a manually movable and rotatable marker on the monitor screen that displays an ultrasound scan image. The operator can move and rotate the marker through manual control to visually align the marker in parallel with the extending direction of the blood vessel where the measurement point is located. The angle between the marker and the ultrasound beam is then taken as the Doppler angle .theta.. This marker method, however, has the following two drawbacks.
First, since the blood vessels in a human body are mostly curved and extend in all directions with very few straight segments, the manual control of the marker would be very difficult to achieve precise alignment with the blood vessel. The blood-flow velocity measurement can therefore be imprecise. This drawback is schematically depicted in FIG. 1B. As shown, if the measurement point P is located in a curved segment of the blood vessel 20, then it would be highly difficult for the operator to visually align the marker in parallel with the blood-flow velocity vector .nu. at the measurement point P. The result of the blood-flow velocity measurement is therefore highly untrustworthy.
Second, in a 2-D (two-dimensional) ultrasound scan image, the marker method to find the Doppler angle would be unfeasible if the blood vessel 20 is unparalleled to the scanning plane. To make alignment possible, the scanning plane should be first aligned in parallel with the extending direction of the blood vessel 20. This requirement, however, is difficult to achieve for most internal blood vessels.
If the obtained Doppler angle is imprecise, the subsequently obtained blood-flow velocity .nu.0 from Eq. (A3) will be also imprecise. A conventional solution to this problem involves the use of two sets of ultrasound transducers for 2-D Doppler angle measurement, and three sets for 3-D Doppler angle measurement. This solution, however, is quite complex in system configuration and costly to implement due to the need to use two or more sets of ultrasound transducers.
Through research, Newhouse et al. have found that, if a focusable ultrasound beam with a circular emitting plane (i.e., the ultrasound beam is conically shaped and symmetrical in form about its propagation axis) is used as the ultrasound source, then the band-width B.sub.d of the Doppler spectrum can be formulated as follows: ##EQU4## where .nu. is the blood-flow velocity at the measurement point;
Theoretically, the maximum Doppler frequency .function..sub.max in the Doppler spectrum is defined as the frequency component at the upper bound of the bandwidth B.sub.d and which is equal to the peak-intensity Doppler frequency .function..sub.peak plus half of the bandwidth B.sub.d, i.e.,. EQU .function..sub.max =.function..sub.peak +B.sub.d /2 (A5)
Moreover, Newhouse and Tortoli have jointly found that the maximum Doppler frequency .function..sub.max can be formulated as follows: ##EQU5##
The relationship of Eq. (A6) is herein and hereinafter referred to as "Newhouse-Tortoli Maximum Doppler Frequency Equation" throughout this specification.
Detailed discussions about the equations Eqs. (A4), (A5) and (A6) can be found in the following technical publications:
(1) "Three-dimensional Vector Flow Estimation Using Two Transducers and Spectral Width", IEEE Trans. Ultra. Ferro. Freq. Con., Vol. 41, pp.90-95, 1994, by V. L. Newhouse, K. S. Dickerson, D. Cathignol, and J. Y. Chapelon; PA0 (2) "Ultrasound Doppler Probing of Flows Transverse with Respect to Beam Axis", IEEE Trans. Biomed. Eng., Vol. BME-34, pp.779-789, 1987, by V. L. Newhouse, D. Censor, T. Vontz, J. A. Cisneros, and B. B Goldberg; and PA0 (3) "Theory of Ultrasound Doppler-Spectral Velocimetry for Arbitrary Beam and Flow Configurations", IEEE Trans. Biomed. Eng., Vol. BME-35, pp.740-751, 1988, by D. Censor, V. L. Newhouse, and T. Vontz.
Based on the findings disclosed in these papers, the inventors propose a new ultrasound method and system for measuring the velocity and direction of a flow, such as a blood flow in a human body.